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Are we ready for a 42V automotive system?

Posted: 16 Jul 2004 ?? ?Print Version ?Bookmark and Share

Keywords:automotive? automobile? car? engine? battery?

In the mid-1950s, automobiles were mainly made up of little electrical components: a chassis driven by a combustible engine, an electric starter to crank-start the engine, an alternator to recharge the battery, a step-up coil transformer for ignition and some lights for road guidance. The 12Vdc battery system used then has since been seen as a standard in the automotive industry.

In recent years, ever-increasing demands on environmental, safety, comfort and performance aspects led to the creation of many functions that can only be accomplished with intelligent electrical management. To add control precision, mechanical, hydraulic and pneumatic drives are replaced by electromechanical devices.

A typical passenger Sedan nowadays must be able to handle 1kW at full load. Higher-end luxury vehicles have continuous loads that will make up 2.8kW if they are all exercised at the same time. On a 14V charging system, this equates to 200A of current being handled by the alternator and battery cables. With increasing growth in electrical content and suggestions that power demanding e-motors to replace traditional mechanical drives, the capability of the 12V system is reaching its efficiency limit.

As automotive experts realize the need for a solution to the increasing power demands of future vehicles, a 42V standard is continuously being discussed.

One of the biggest concerns for higher power requirement is the wiring size needed to carry the proportionally higher currents from the power source to the loads. With a 42V source, a current three times less is required to run through the wiring harness when compared to the conventional 12V. Furthermore, power losses due to cable resistance will be reduced in factors of the square of the current flowing through it. Being able to downsize the cable gauge and having smaller wire bundles will lead to appreciable benefits to the automotive industry. Manufacturability wise, the routing and installation of smaller wiring harness can be simpler and faster. Multiplexing architectures would be part of the winners as supplying various loads with power through single-wire busing will be less of an issue.

The 42V will also accelerate the introduction of e-drive motors to reap the benefits of better fuel consumption and performance. Electrically-driven starter alternator, AC compressor, interior air heater, catalyst heater, oil pump, water pump, turbo and the much talked about x-by-wire technology for brakes, suspension and power steering are now more practical as these high-power requirement devices can be easily supplied with substantial energy.

Challenges for going 42V

Most automotive components built today are designed only for 12V operation. Bulbs, electromagnetic valves, DC motors, ignition coils, heaters and other passive devices will not work on 42V correctly without modifications. It would take investments from all parties to adapt their products to this higher voltage.

Technically, a voltage at 42V will see arcing at mechanically switched contact points. Such arcing, if not controlled either by changing contact points material or altering the opening speed and switch gap, can disastrously wear off the switch in a short period. Mechanical relays, switches, fuses, connectors and brush DC motors will face intense pressure in finding a cost-effective solution.

Although it is possible to make devices rated for 42V operation, it is known that bulbs and filament lamps work better with 12V. For a filament to be as bright in 42V, it has to be made thinner for higher resistance. During operation, thin flimsy filaments are susceptible to vibration and shock, which will lead them to break, thus failing the lamp. Till the time LEDs or high-intensity gas discharge lamps take over bulbs for illumination, there will always be this argument of not going 42V-only.

From an electrical systems perspective, a redesign is required to relayout the power distribution, which includes fusing and cable size gauging. As the system charging voltage climbs to this higher value, load dump characteristics of the alternator will also change. While many current 12V electronic module designs cater for load dump protection, they may not survive transient voltage spikes in the 42V charging system. System designers will need to evaluate going for a robust centralized load dump protection at the alternator, as this could avoid putting in costly measures to handle the higher voltage surges at each electronic module.

Power semiconductors will need higher dielectric strength than what is commonly designed for the 12V application. Typical built-in protection in power devices for inductive loads generally has clamping in the 60V range. With 42V type loads, the protection has to withstand a higher clamping voltage in the 75V region. With this clamping voltage defined, the device operation range of power semiconductors can be set.

Gearing up for 42V

Switching directly from a 12V to a full-blown 42V system overnight is not realistic. The infrastructure of supporting components has to be available before consumers would commit in buying a car with this power network. It is often an edgy gamble and test of courage for developers to invest in emerging markets. On the contrary, being the first few pioneers having the lead will allow one to reap benefits in terms of profit, technology and experience.

Carmakers are recommending having an intermediate dual voltage 12V/42V transition stage. The first Sedan ever launched having a 42V net onboard was in 2001 with the Toyota Crown Royal. It used the 42V secondary source to drive an auxiliary starter-generator, which is also able to drive the air compressor while the engine is off. Soon, the second car will be an SUV from General Motors Corp. It will also incorporate a 115V AC power outlet supplied by 42V input voltage. Mass production of the vehicle will start this year.

While there were two consortiums formed in 1996 to agree upon guidelines for the 42V net, the SICAN Forum, also known as Vehicle Electrical Systems Architecture Forum, is the brainchild of the existing standards. Though standards were defined, the implementation is left to the vehicle OEMs and component suppliers to decide. There are no clear-cut directions on how the transition toward 42V should be done.

For many tier-1 electronic control unit suppliers, they have started to design electronic modules with internal power supply circuits to accept 42V. Additional protection in both input and output stages of the electronic module would be necessary. Input-voltage clamps need to handle higher-voltage spikes, power devices will have to handle switching of higher voltages, and avalanche breakdown voltages have to be redefined. A DC/DC converter is usually present to step down the input voltage to 5V or 3.3V for running the digital circuitry. Power output drivers must be upgraded to switch the incoming 42V supply rail.

For system-level OEMs, many are starting to decide ways to either prevent or cater for accidental reverse-battery connection, wrong replacement of 42V devices with 12V-rated devices, compatibility of multiplexing protocols physical layer and redefining standby mode current.

- Wong Sou Loong

Infineon Technologies AG

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